Patent Document 1 and Patent Document 2, for example, describe a nonaqueous electrolyte secondary battery that has, between a negative electrode and a positive electrode thereof, a porous protective film containing a resin binder and an inorganic oxide filler. With such structure of this nonaqueous electrolyte secondary battery having a porous protective film, the occurrence of an internal short circuit is prevented even after an active material that falls off from an electrode or a waste from a cutting process adheres to the surface of the electrode at the time of manufacture of the nonaqueous electrolyte secondary battery. However, the problem of this structure is that if an internal short circuit occurs, the occurrence of the internal short circuit cannot be detected with a conventional method that uses a cell of a conventional structure without a porous protective film.
In order to explain this problem, first the conventional method that uses a cell of a conventional structure without a porous protective film is described hereinafter.
Specifically, in the case of the cell of a conventional structure without a porous protective film, when an internal short circuit occurs, the voltage of the cell drops drastically as shown in FIG. 3, and does not recover. Therefore, the internal short circuit can be detected by monitoring the voltage of the cell on a predetermined cycle or detecting a drastic increase in temperature caused by a short-circuit current.
This is caused by the following mechanism. For example, when, as shown in FIG. 4A, an internal short circuit has occurred by a metallic foreign body such as an electrode material or waste that falls off during the manufacturing process, a positive electrode aluminum core of a short-circuit part is melted by the heat generated by the short circuit, as shown in FIG. 4B. Subsequently, this heat melts and contracts the separator made of polyethylene or other high-polymer material, as shown in FIG. 4C, and enlarges the short-circuit hole as shown in FIG. 4D, whereby the short-circuit area increases. Thereafter, the short-circuit section melts, as shown in FIG. 4(e), and thus generated heat repeatedly enlarges the molten section (short-circuit hole) again, as shown in FIG. 4C. In this manner, the voltage of the cell drops drastically and the temperature of the cell increases drastically due to thermal runaway.
Patent Document 3, for example, describes that when the temperature of the battery rises due to an internal short circuit, the rise of the temperature is stored, thereby enabling a detection of an internal short circuit or the like that occurs when the battery is not operated. Patent Document 3 further describes that when a significant temperature increase with respect to a significant voltage drop is detected, it is determined that an internal short circuit has occurred. Patent Document 4 describes that an internal short circuit is detected from voltage, pressure, temperature, sound, and the like. Further, Patent Document 5 discloses that signals having a plurality of frequencies are applied from an electrode to detect an internal short circuit.    Patent Document 1: Japanese Patent Application No. 3371301    Patent Document 2: International Publication WO05/098997 Pamphlet    Patent Document 3: Japanese Unexamined Patent Application No. 118-83630    Patent Document 4: Japanese Unexamined Patent Application No. 2002-8631    Patent Document 5: Japanese Unexamined Patent Application No. 2003-317810
On the other hand, in the structure with a porous protective film as described in Patent Document 1 or Patent Document 2, when an internal short circuit occurs by a metallic foreign body such as an electrode material or waste that falls off during the manufacturing process as shown in FIG. 5A, the following case occurs. Specifically, even when the positive electrode aluminum core of the short-circuit part is melted as shown in FIG. 5B, the porous protective film prevents the positive electrode aluminum core and a negative electrode binder from coming into contact with each other. For this reason, the separator melts only in the vicinity of the region where the metallic foreign body exists, as shown in FIG. 5B to FIG. 5D, whereby the expansion of the short circuit is prevented. Thereafter, the voltage of the cell substantially recovers, and the cell can be used with a small short circuit. FIG. 6 shows changes in the voltage of the cell that occur when an internal short circuit occurs in the structure described in Patent Document 1 or Patent Document 2. The problem of the methods described in Patent Documents 3 to 5 is, therefore, that it is difficult to detect an internal short circuit.
Because a secondary battery that uses olivine type lithium iron phosphate (LiFePO4) as a positive electrode material has high thermal/chemical stability and is inexpensive, this secondary battery is expected to be an alternative to a secondary battery that uses lithium cobalt oxide (LiCoO2). However, because the secondary battery that uses olivine type lithium iron phosphate (LiFePO4) as the positive electrode material has a low electrical conductivity and an extremely low lithium ion diffusion rate, the methods described in Patent Documents 3 to 5 cannot detect an internal short circuit, which is the same problem as that of the secondary battery with a porous protective film that is described in Patent Document 1 or Patent Document 2.